Receiver protection method and apparatus

ABSTRACT

A multipactor receiver protection stage is provided with a soft beta emitter, such as a tritium doped material, for decreasing the turn-on time of the stage by providing free electrons in the gap between the secondary emission surfaces of the multipactor for acceleration by electromagnetic wave energy within the multipactor cavity.

United States Patent 1191 Nelson et al.

[ 1 RECEIVER PROTECTION METHOD AND APPARATUS [75] Inventors: Theodore M. Nelson, Catonsville;

Harry Goldie, Randallstown, both of Md.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: July 31, 1972 [211 App]. No.: 276,346

521 11s. c1 333/13, 313/54, 313/104, 333/98 MP 51 1111. c1 HOlp 1/14,H01j 7/70, l-lOlj 43/02 158 Field of Search 333/13, 98 R, 98 MP; 313/54, 103405; 315/39 [56] References Cited UNITED STATES PATENTS 3,278,865 10/1966 Forrer 333/13 Dec. 31, 1974 3,309,561 3/1967 Kane 315/39 3,354,349 11/1967 Horn 3,705,319 12/1972 Goldie et a1. 333/13 X FOREIGN PATENTS OR APPLICATIONS 851,881 10/1960 Great Britain 333/13 1,186,897 9/1959 France 333/13 Primary E.\'aminer-Archie R. Borchelt Assistant Examiner-Marvin Nussbaum Attorney, Agent, or Firm-J. B. Hinson [57] ABSTRACT A multipactor receiver protection stage is provided with a soft beta emitter, such as a tritium doped material, for decreasing the turn-on time of the stage by providing free electrons in the gap between the secondary emission surfaces of the multipactor for acceleration by electromagnetic wave energy within the multipactor cavity.

3 Claims, 6 Drawing Figures lNDUCT/VE TUNING MEMBERS 35 ,SECONDARY EMISSION SURF 3a ACUATED CHAMBER 92 PATEIITEII m I IQII 3858.125

SHEET 10F 2 FIG.1

28 I I I2 I8 A,

SOLID STATE GAS MEDIUM HIGH R LIMITER IGNITER POWER POWER STAGE STAGE STAGE STAGE LIL} IIIIDucTII/E TUNING SECON MEMBERS 3'5 EMISSION SURF (ES 38 aq/ EVACUATED CHAMBER a2 RECEIVER PROTECTION METHOD AND APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to receiver protection apparatus and more particularly to a high power receiver pro tection stage interposed in a waveguide section between a receiver and its antenna.

2. State of the Prior Art In various kinds of radar systems, receiver protectors are interposed between a receiver and its antenna to prevent transmitted radiation reflected by the antenna (or intense direct radiation) from damaging receiver elements. Transmit/receiver (T/R) tubes have been utilized to protect the receiver from such high energy radiation. T/R tubes in general, however, lack the capacity for recovery and for a turn-on time of sufficiently short duration to adequately protect the receiver in many pulsed radar applications, such as pulse doppler radar.

Circulators have been substituted for T/R tubes by which the majority of the energy from a transmitted pulse is prevented from reaching the receiver. Even with circulators, a portion of the transmitted energy is reflected back from the antenna through the circulator to the receiver. To prevent this reflected energy from reaching the receiver, a high power protection stage is often coupled into the microwave transmission line between the antenna and the receiver. This high power protection stage may include a vacuum-enclosed, high gap formed between parallel opposed secondary emission surfaces on opposing electrodes within a sealed evacuated microwave cavity section. This type of protection device is generally referred to as a multipactor.

The introduction of electromagnetic wave energy into the multipactor cavity accelerates free electrons in the gap back and forth between the secondary emission surfaces of the electrodes in such a manner that multiplicative electron production results from the impacting of these electrons on the secondary emission surfaces. The flow of electrons between the secondary emission surfaces resulting from this electron multiplication defines the microwave cavity and reflects the incoming energy to thereby limit the amount of electromagnetic energy that reaches the receiver.

Multipactors of the type described supra are characterized in general by extremely fast recovery times, on the order of of 1 radiofrequency (r.f.) cycle, e.g., less than 1 nano-second even at L band frequencies. When the gap sapcing satisfies the multipactor equation, as hereinafter described, electron travel through the electrode gap is in phase with the r.f. voltage so that electron build-up will continue. When the r.f. signal disappears, the electrons are no longer accelerated .across the gap and are quickly dispersed in the high vacuum, thus accounting for the fast recovery time. The multipactor design is, therefore, especially well suited to be used as a front end stage of a receiver protector because of the nanosecond recovery times attainable therewith.

One major difficulty with multipactor protection stages is the unusually long turn-on times, which limits their application to pulsed radars. In one reported case, for instance, as many as four r.f. pulses were transmitted to a receiver before multipactor flow was initiated.

As is readily appreciated, four r.f. pulses may carry enough power to burn out the receiver, especially when a crystal detector is located at the terminus of a waveguide. The long turn-on time is due, at least in part, to a scarcity of free electrons within the gap between the secondary emission elements.

One successful solution to this slow turn-on problem has been the use of an electron gun to provide a ready source of electrons. However, the use of an electron gun necessitates external power connections and in addition requires that the protectors be continuously in operation to be effective.

SUMMARY OF THE INVENTION It is accordingly an object of this invention to provide an improved, fast acting, high power receiver protector stage which is continuously in operation without external power application and in which the turn-on time of the high power stage is sufficiently short to prevent receiver damage.

It is another object of this invention to provide a method and apparatus for improving the response time of a high power receiver protection stage.

It is a further object of this invention to provide an improved, safe method and apparatus for decreasing the on-time characteristic of a multipactor type high power receiver protection stage.

It is a still further object of this invention to provide improved apparatus for protecting a wave energy receiver from damage by high energy level electromagnetic pulses.

Briefly, in accordance with the present invention, a soft beta emitter is located in the evacuated cavity of a multipactor type receiver protection stage to provide free electrons in the gap between the secondary emission surfaces thereof. The production of free electrons of sufficient quantity in the electrode gap reduces the turn-on time of the protection stage to less than 1 nanosecond without radiation hazard danger. More particularly, the soft beta emitter may be a tritium doped material located in close proximity to the electrode gap, and may extend from the cavitys inductive tuning members to minimize the effect of the soft beta emitter apparatus on cavity resonance characteristics. The beta emitter continuously produces free electrons without the application of external power so that high energy level pulses will be prevented from damaging receiving apparatus, regardless of the availability of electrical power.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 comprises a simplified schematic, in block diagram form, of a receiver protector including a high power front end stage;

FIG. 2 comprises a sectional view of the front end stage of FIG. 1 and shows a multipactor with a soft beta emitter in close proximity to the gap between the secondary emission surfaces of the multipactor;

FIG. 3 comprises a graph of the amplitude of the output signal from the multipactor of FIG. 2 as a function of time; and

FIGS. 4, 5 and 6 comprise perspective views showing the alternative placement and configuration of a soft beta emitter relative to the secondary emission electrodes.

DETAILED DESCRIPTION Referring to FIG. 1, a microwave transmitter 10 and a microwave receiver 12 are connected to a conventional circulator 14 which is in turn connected to an antenna 16. Receiver protector apparatus 18 is interposed in the transmission path between the antenna 16 and the receiver 12.

The receiver protector apparatus 18 of FIG. 1 in one embodiment may be a waveguide element containing a number of serially connected receiver protection stages. At the front end of this waveguide element is a high power stage 20, which may be of the aforementioned multipactor type. This high power stage may be followed by a medium power stage 22, a gas igniter stage 24, and a solid state limiter stage 26, all of con ventional design, for preventing signals of successively decreasing power from reaching the receiver 12. The medium power and igniter stages are gas filled to utilize ionic transport mechanisms for receiver protection.

In operation, a pulse from the transmitter 10 travels through the circulator 14 to the antenna-16 in the direction illustrated by the arrow'28. The pulse comprises a high frequency sinusoidal electromagnetic wave having a characteristic .E or electric vector which oscillates between opposing orientations at a frequency equal to the frequency of the wave. The pulse is applied from the circulator 14 to the antenna 16 from whichit is radiated into free space.

However, there is a portion of each transmitter pulse which is reflected back to the circulator 14 from the antenna. This reflected portion of the pulse may contain to per cent of the energy of the pulse and may range in power from 100 watts to hundreds of kilowatts. The reflected pulse traverses the circulator 14 in the direction shown in FIG. 1 by the arrow 30 and is applied to the front end of the protection apparatus 18.

As shown in section in FIG. 2, the high power stage of FIG. 1 may be of the multipactor type in which the reflected pulse is introduced into an evacuated chamber 32 of a waveguide 34. A pair of electrodes 36 have secondary emission elements with opposing parallel surfaces 38 which define a vacuum-enclosed high Q gap, with a gap separation d satisfying the multipactor equation:

where e/m is the charge to mass ratio of the electron;

V is the peak r.f. voltage of the reflected pulse;

f is the frequency of the pulse; and

n is an integer indicating the mode of operation of the waveguide section.

The multipactor gap, d, may be adjustable by threaded adjustment means 44, in which electrode portions pass through a flexible diaphragm 46, used for mechanical stability.

The electron multiplying, secondary emission material may be any suitable material such as copper which is generally preferable because its introduction into the microwave cavity minimizes insertion losses.

As explained supra, the generation of electron currents between the electrodes 36 is predicated upon the existence of free electrons in the electrode gap and the turn-on time of the multipactor in response to the reflected pulse will be decreased as a function of the number of free electrons available in the electrode gap. In the present invention and as illustrated in FIG. 2, these free electrons may be provided by a soft beta emitter 40 carried by an inductive tuning member 35 which passes through the waveguide 34 wall to tune the cavity.

In one embodiment, this beta emitter may be a tritium doped element which is commercially available from the US. Radium Corporation, of Bloomsburg, Pa. The emitter 40 continually provides a large number of free electrons for r.f. acceleration to the surfaces 38 of the secondary emitter elements. In one form, tritium is obtainable on a thin metal foil in the form of an ahydride. Up to 150 millicuries may be allowed by the Atomic Energy Commision per receiver protection unit. Since 50 millicuries are generally sufficient for the back stages of the protector, the front stage 20 may utilized millicuries.

As will be subsequently explained, a 100 millicurie source provides enough free electrons to decrease the turn-on time of a multipactor to less than 1 nanosecond without radiation hazard. One hundred millicuries corresponds to 3.7 X 10 disintegrations per second, and measurements indicate that one-fourth of the electrons generated by these disintegrations are emitted in a forward direction. About 5 X 10 electrons per second or 500 per microsecond can be produced in the electrode gap between the secondary emission surfaces 38 by proper focusing of the emitted electrons with thin, flat emitting surfaces facing the gap.

From the above it can be seen that there will exist at least one free electron between the secondary emission surfaces 38 every 2 nanoseconds, or every two to four r.f. cycles. Because of the direction of emission of the free electrons towards the gap, the free electron is very likely to strike the secondary emission material even when there is no wave energy in the cavity. This produces a more than adequate supply of electrons with a small amount of radioactive material. Many priming or free electrons are thus present in the gap and fast turnon of the multipactor discharge is assured, even for very short r.f. pulses, safely, and without the use of external power or power connections.

Assuming at least one free electron in the gap between the surfaces 38 when electromagnetic wave energy of an appropriate frequency and energy level, such as that contained in the reflected portion of a transmitted pulse, is applied to the cavity 32, the free electron is accelerated by the E vector of the electromagnetic wave towards one of the surfaces 38. If the secondary emission material has an emission factor greater than 1, the bombardment of the material produces a number of electrons. The electric vector reverses direction as these additional electrons are produced and the electrons are accelerated towards the other of the secondary emission surfaces 38. The effect is multiplicative and a large alternating current flow is set up between the electrodes 36. This current flow not only detunes the cavity 32, but also acts as an effective short circuit to reflect the applied energy.

With the use of a soft beta emitter, the output signal from the high power stage 20 of FIG. 1 is initially a narrow pulse 42, such as illustrated as the pulse shown in FIG. 3. With continued reference to FIG. 3, the high amplitude portion of the pulse 42 is narrow in width,

indicating a turn-on time I which is only a fraction (2/4 r.f. cycles) of the total duration t of the pulse. The remainder of the electromagnetic energy from the high power stage of FIG. 1 is thus lowered to a level where it can be easily dissipated by the succeeding stages 22-26 prior to reaching the receiver 12.

Referring now to FIGS. 4, 5, and 6 where alternative embodiments of the electrodes 36 of FIG. 1 are illustrated, the tritium doped element of FIG. 4 may take the form of a wand 48 having a tip 40 of tritium. Alternatively, as shown in FIG. 5, one of the electrodes 36 may be provided with a frustoconical shoulder 50 which has been tritium doped. A further embodiment of the electrode 36 is shown in FIG. 6 where tritium doped flags 52 provide the free electrons.

The soft beta emitters of FIGS. 4 and 6 may be mounted in the waveguide cavity 32 without disruption of the propagation characteristics of the waveguide by locating the emitter on, at, or through the inductive tuning member shown in FIG. 2, generally used to tune out the capacitance introduced by the multipactor electrodes 36.

In each of the embodiments of the electrodes 36 illustrated in FIGS. 4, 5 and 6, the tritium doped material is quite thin so that free electrons produced in the material will not be absorbed before they can emerge therefrom. This also serves the aforementioned focusing function in that the larger the flat area exposed to the electrode gap, the larger will be the proportion of total emitted electrons reaching the gap. The spacing of the soft beta emitters from the gap between the electrodes 36 is not critical so long as a number of priming or free electrons reach the electrode gap. An optimum spacing of I milli-inches between the electrode gap and the beta emitter is preferred, with practical emitter gap spacings ranging from about 0.1 to 0.25 inches. Other spacings, however, may be used depending on emitter configuration and the abovepreferred gap to emitter spacings are not limiting.

While tritium doped material has been described, it will be understood that all beta emitting materials are included within the scope of this invention, with the soft beta emitting materials such as the commercially available nickel isotope 63 being preferred for safety in handling and operation.

From equation (1), supra, it can be seen that multipacting is a function of both frequency and peak wave amplitude. For a predetermined electrode gap spacing d, a fairly wide range of incoming pulse frequencies and power levels can be accommodated. While maximum efficiency of electron multiplication is accomplished at frequencies and power levels set by the multipactor equation, multipacting operation will occur, although with less efficiency, at other than the specific frequency and power level as determined by the equation. It will be appreciated that multipactors are usually used in systems in which frequency and power levels are relatively stable. Thus, in a radar operating at a predetermined frequency and power level, a single pair of electrodes with a fixed gap spacing will normally suffice. However, by utilizing multiple electrode pairs within the same evacuated cavity with graduated electrode 10 fast turn-on time provided by the soft beta emitter. The

protection stage needs no external power, is active to protect the receiver even when all powerto the radar has been turned off, and fast action is obtained without a radiation hazard.

15 What is claimed is:

1. Apparatus for protecting a wave energy receiver from damage by high energy level pulses comprising:

a. a wave guide element adapted to be positioned in a transmission path along which wave energy is directed to the wave energy receiver, said element including an evacuated chamber through which the wave energy is directed to the receiver;

b. a pair of frustoconical shaped electrodes positioned in said chamber, at least one of said electrodes including an annular tritium doped shoulder and an electron multiplying surface spaced from said shoulder, said electrodes being positioned on opposite sides of said chamber.

2. Apparatus for protecting a wave energy receiver from damage by high level pulses, comprising:

a. a wave guide element adapted to be positioned in a transmission path along which wave energy is directed to the wave energy receiver, said element including an evacuated chamber through which the wave energy is directed to the receiver;

b. means for controlling the passage of wave energy through said chamber, said means including first and second electrodes positioned on opposite sides by said chamber to form a multipactor gap; and

c. inductive tuning members including a tritium doped electron source means disposed adjacent said multipactor gap for tuning said multipactor gap and for supplying an initial source of electrons therein.

3. Apparatus for protecting a wave energy receiver from damage by high level wave energy pulses comprising:

a. a wave guide element adapted to be positioned in a transmission path along which said wave energy is directed to said receiver, said element including an evacuated chamber through which said wave energy is directed to said receiver;

b. means including a pair of electrodes having secondary electron emitting properties, said electrodes being positioned in said chamber to form a gap for controlling the passage of energy through said chamber; and

c. means for tuning the gap formed by said elec trodes, said tuning means including a radioactive source for injecting electrons into said gap. 

1. Apparatus for protecting a wave energy receiver from damage by high energy level pulses comprising: a. a wave guide element adapted to be positioned in a transmission path along which wave energy is directed to the wave energy receiver, said element including an evacuated chamber through which the wave energy is directed to the receiver; b. a pair of frustoconical shaped electrodes positioned in said chamber, at least one of said electrodes including an annular tritium doped shoulder and an electron multiplying surface spaced from said shoulder, said electrodes being positioned on opposite sides of said chamber.
 2. Apparatus for protecting a wave energy receiver from damage by high level pulses, comprising: a. a wave guide element adapted to be positioned in a transmission path along which wave energy is directed to the wave energy receiver, said element including an evacuated chamber through which the wave energy is directed to the receiver; b. means for controlling the passage of wave energy through said chamber, said means including first and second electrodes positioned on opposite sides by said chamber to form a multipactor gap; and c. inductive tuning members including a tritium doped electron source means disposed adjacent said multipactor gap for tuning said multipactor gap and for supplying an initial source of electrons therein.
 3. Apparatus for protecting a wave energy receiver from damage by high level wave energy pulses comprising: a. a wave guide element adapted to be positioned in a transmission path along which said wave energy is directed to said receiver, said element including an evacuated chamber through which said wave energy is directed to said receiver; b. means including a pair of electrodes having sEcondary electron emitting properties, said electrodes being positioned in said chamber to form a gap for controlling the passage of energy through said chamber; and c. means for tuning the gap formed by said electrodes, said tuning means including a radioactive source for injecting electrons into said gap. 